Glad your brakes got better. I'd say it's normal that the ds2500 doesn't have such a great bite initially. But if you're aware of it and warm them up a little each time before you gun it, it's fine. Doesn't really take much, according to my experience.
As for brake fluid: First of all, that should be dot 5.1, not dot 5. Dot 5 is silicone based and is not compatible with dot 3, dot 4 or dot 5.1. Therefore, it should only be used if you completely rebuild the hydraulic system. But I'm sure you don't want to do that. Also, dot 5 fluid, unlike dot 4 or dot 5.1, doesn't absorb water. That sounds like an advantage, but as far as safety goes, it really isn't. So, as long as you don't get water in the system, it's fine, but if you do, it's dangerous. Don't know about the UK, but in Germany, it's illegal to use dot 5 on the road. And in any case, dot 5 should not be used with ABS braking systems, afaik.
So, a high quality dot 4 should actually be sufficient. I'm using ATE Super Blue Racing which is a dot 4 specification but does have a dry boiling point of 280°C and of 198°C wet.